When burned, hydrogen does not produce carbon dioxide, a notable greenhouse gas, and it ends up as water after it has released energy. For this reason, hydrogen has attracted the greatest attention as a source of clean energy. Traditionally, hydrogen has been produced from water by processes such as electrolysis of water, thermal decomposition of fossil fuels (e.g., heat decomposition of coal and reduction of water vapor by coal, oil, or gaseous hydrocarbons), and thermal decomposition of methanol.
However, the processes that rely on fossil fuels (e.g., oil, coal, and LNG) have limited future potential when the natural resources are in threat of being exhausted. In addition, these processes produce carbon monoxide and carbon dioxide along with hydrogen, and the resulting products contain significant amounts of impurities derived from fossil fuels. Thus, much effort has been put into developing processes for producing hydrogen from water, a material that does not pose any of these problems. However, only few water-based methods are available other than the electrolysis, including thermochemical decomposition, in which water is thermally decomposed at 800° C. or higher temperatures and which involves multiple chemical reactions known as the UT-3 cycle and IS process, and solar energy processes, which utilize photocatalysts or microorganisms. Many of these processes are still at the stage of basic research and have significant problems, such as very low yield and corrosion of the apparatuses.
Water in its subcritical or supercritical conditions exhibits different properties than when it is under normal conditions. By taking advantage of these properties, several production processes of hydrogen have been developed. These processes, however, are nothing more than the replacement of water in conventional water-based hydrogen production processes, which is used under normal temperature and pressure conditions, with subcritical or supercritical water.
For example, Japanese Patent Laid-Open Publication No. Hei 11-279782, entitled “Process for producing high-pressure hydrogen gas”, describes a process in which water for electrolysis is subcritical or supercritical water. Japanese Patent Laid-Open Publication No. Hei 11-278801, entitled “Process for producing hydrogen gas”, describes a process in which a high-pressure hydrogen gas is obtained by dispersing a light-scattering semi-conductor powder in subcritical or supercritical water and irradiating light to cause the water to decompose. Also, a production process of hydrogen is described in Japanese Patent No. 2979149 in which a carbon-containing substance is reacted with supercritical water at 600° C. or a higher temperature to cause the water to thermally decompose.
Conventional hydrogen-production processes involving the use of subcritical or supercritical water are based on the assumption that the reaction of the water with any organic compound is an oxidation reaction. These processes are therefore focused on the promotion of the oxidation of carbon atoms by water and each requires the use of a metal catalyst and supply of oxygen.
In fact, production process of hydrogen using subcritical or supercritical water is always associated with the generation of carbon dioxide, evidencing that the reaction is an oxidation process. One example is a process described in Japanese Patent Laid-Open Publication No. 2000-239672, in which a carbon source is reacted with subcritical or supercritical water to cause thermal decomposition and hydrolysis of the carbon source and to thereby give a light gas. This process requires collecting the carbon dioxide by-product. Another example is a proposal described in Japanese Patent Laid-Open Publication No. 2000-143202, which involves the use of a carbon dioxide absorbent. None of these approaches rely on a carbon dioxide-free reaction for hydrogen production.
The present inventors intensively studied the behavior of the atoms in water molecules in their subcritical or supercritical conditions and found that when water undergoes phase transition to the supercritical state in the presence of an alcohol compound, it produces water-derived hydrogen without generating carbon dioxide and the alcohol compound is predominantly converted into a corresponding carbonyl compound, rather than into a corresponding carboxylic acid.
In other words, the present inventors have found that a direct oxidation/reduction process can take place between an organic compound and subcritical or supercritical water.
This finding led the present inventors to devise the present invention.
Accordingly, the present invention in one aspect provides a process for effectively generating/producing water-derived hydrogen from supercritical water without generating carbon dioxide by-product.
In another aspect, the present invention provides a simple process for producing a carbonyl compound from an alcohol compound through a reaction of subcritical or supercritical water with the alcohol compound.
In known processes, an alcohol compound is converted to a carbonyl compound via oxidation of alcoholic hydroxyl groups into carbonyl groups. The oxidation process must be carefully controlled since strong oxidation may result in oxidative cleavage of the alcohol or generation of a carboxylic acid.
In carrying out these processes, the conditions for oxidation, including the types of the oxidizing agent and the solvent used and high temperature control, must be properly adjusted to avoid side reactions since the tendency of the alcoholic hydroxyl group to be oxidized varies depending on the type of the functional group adjacent to the carbon atom bearing the hydroxyl group. In addition, many of the oxidizing agents and the catalysts contain heavy metals and are difficult to handle.
The process of the present invention is characterized in that a carbonyl compound can be obtained from a corresponding alcohol compound without requiring the use of heavy metal oxidizing agents or catalysts and without undesirable side reactions.